Some changes in API

* Replace 'initialize' method of an application with constructor. This method was necessary to use shared_from_this with applications, but it's not supported any more. User must create his own state owned by shared_ptr if he wants. Usualy it's not necessary, because framework stopes generate events before the application object die.
* Pass dispatch to application and application to handlers with reference, not pointer.
* 'map' method of generator automaticaly unwraps argument. For example, map of generator<future<int>> will call callback with future<int>, not future<future<int>>.
* 'get_service_async' of service_manager_t returns service immediately, because now services can accept calls while connecting. Before services couldn't and 'get_service_async' returned future<Service>. Note that 'get_service' also returns service, but it synchronously waits for connection.